Adverse birth outcomes among nulliparous vs. multiparous women.

OBJECTIVES Previous studies indicate that nulliparous women (i.e., women having no previous births) are at higher risk for adverse birth outcomes than multiparous women (i.e., women having had at least one previous birth). We examined whether part of the difference in adverse outcome rates is attributable to nulliparous women with poor pregnancy outcomes being less likely (through choice or fecundity differences) to have a subsequent live birth within the same time period as nulliparous women without adverse outcomes. METHODS Using deterministic matching, we linked nulliparous women from the North Carolina Detailed Birth Record to subsequent births. We employed statistical and simulation-based analyses to estimate first birth outcome rate differences between nulliparous women who did have a subsequent live birth vs. those who did not. Our Markov simulations focused on preterm birth (PTB). RESULTS Among nulliparous women who were not linked to a second birth, maternal age-adjusted rates of multiple adverse outcomes were all statistically higher compared with rates for linked women. These results also held in race/ethnicity-specific analyses. Simulations found that the relative risk of PTB associated with a history of PTB was underestimated if some women who would have been at risk for PTB did not experience a second birth. CONCLUSIONS The observed differences in rates of adverse outcomes between nulliparous and multiparous women are partly attributable to higher-risk women not having a subsequent live birth, either by choice or due to fecundity differences.